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Previous heating cycles had shown this increased tremor to last for days to weeks."As part of our normal routine monitoring, we aim to visit Te Wai a-moe this week to collect water and gas samples from the lake," he said.

"The Crater Lake has undergone many heating and cooling cycles in the past and we don't see any unusual signs of increased unrest."

The Aviation Colour Code for Mt Ruapehu also remained at Green, the normal level."While this is the case, it is a useful reminder that eruptions can occur with little or no warning."

The last time Ruapehu erupted was on September 25, 2007, causing a seven-minute-long earthquake, two lahars and flying rocks - one which crushed the leg of primary school teacher William Pike when it landed on Dome Shelter near the crater.